1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to an apparatus for continuously supplying lithographic printing material to the plate cylinder of a planographic printing press or a plate-material imager.
2. Description of the Related Art
Traditional techniques of printing an image onto a recording medium, such as paper, include letterpress printing, gravure printing and offset lithography. All of these printing methods require the use of plate material. This plate material is usually loaded onto a rotating plate cylinder that is brought into pressurable contact with the recording/printing medium.
In letterpress printing, the image is represented on the plate material as raised surfaces that accept ink and transfer it onto the medium. Gravure plates, in contrast, define a series of wells or indentations that accept ink for deposit onto the recording medium. Excess ink is removed from the plate material using a doctor blade or another similar device prior to contact between the plate and the recording medium.
In offset lithography, an image is defined on a printing plate defined by ink-accepting (oleophilic) areas surrounded by ink-repellent (oleophobic) surfaces. Two different lithographic systems are generally employed in offset lithography. In a dry printing system, the plate material is simply inked, and the image is transferred onto a recording/printing medium. First, the plate material makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other medium. The paper is typically pinned to an impression cylinder in rolling contact with the blanket cylinder, which applies ink to the paper in accordance with the image.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or “fountain”) solution to the plate material prior to inking. The fountain solution prevents ink from adhering to the non-image areas but does not affect the oleophilic character of the image areas.
Different techniques have been developed for affixing plate material to underlying plate cylinders. Basic offset printing systems involve stationary clamping of a flexible length of plate material to the plate cylinder, while more advanced systems such as those described in U.S. Pat. Nos. 5,355,795 and 5,727,749 (both co-owned with the present application, and expressly incorporated herein by reference) use a relatively long length of plate material or web material stored in the form of rolls within a well or cavity in the plate cylinder. In these systems, a new segment of the plate or web material is advanced around the plate cylinder following completion of a print job. The new segment is imaged by an electronically controlled print head, which applies a print pattern to the surface.
Friction between the web material and the cylinder surface keeps web material stationary relative to the cylinder surface during each print job. As long as the force that tries to move the web does not exceed a critical value of the friction between the web and the cylinder surface, Fc, the web remains stationary against the cylinder surface without slipping. On one hand, a sufficiently high value of Fc is especially important when the same length of the web is printed multiple times, e.g., each time with a different color, to complete an image. Slight movements will cause error in registration, e.g., misalignment between colors. On the other hand, too high an Fc value hinders the smooth and quick advancement of the web material between print jobs. Therefore, an optimal Fc needs to be balanced. Fc is given by the formula Fc=μs×N, where μs is the static friction coefficient and N is the normal force exerted by the web on the cylinder surface.
Meanwhile, there are various forces that may cause the web to slip against the cylinder surface. For example, the web material experiences significant tangential forces as a result of contact with the blanket cylinder, the forces resulting primarily from slight differences in the rolling diameters of the mating cylinder surfaces, which are in contact at sufficient pressure to compress the compliant blanket cylinder surface. These forces tend to alter the orientation of the web material or dislodge it completely unless the critical value of the friction, Fc, remains high enough that it is not exceeded.
Various approaches have been taken to maintain proper friction between the web and the plate cylinder's surface during and between print operations. The static friction coefficient μs is normally constant during printing. The normal force N exerted on the cylinder surface, however, can be changed in order to adjust the friction. One approach is to ensure the amount of the web wrapped around the cylinder remains constant so that the normal force exerted by the web on the cylinder surface remains constant. In the '749 patent mentioned above, a mechanical tensioning mechanism is incorporated into the web-advancing system to ensure that the same amount of web material is dispensed from a supply spool, wrapped around the cylinder circumference, and wound up by an uptake spool. As fresh web material is needed, the uptake spool may be rotated under the action of a clutching motor while a lock (typically a ratchet and pawl assembly) is released on the supply spool to allow fresh web material to be drawn therefrom. After sensing or calculating the radius of at least one of the spools, the device in the '749 patent uses that information to adjust the amount of supply spool rotation allowed by a lock/brake system during each advancing cycle in order to compensate any effect on the amount of web payout that might result from changes in spool radius.
A different approach is to measure or detect changes in the traveling tension of the web and use that information to adjust the normal force exerted against the cylinder. For example, U.S. Patent Application Publication No. 2001/0006029 by Ogawa et al. describes use of an angular displacement arm that swings as the web traveling tension changes. The angular displacement of the arm is used to calculate the traveling tension of the web, which is, in turn, used to adjust rotor speed of the spools. In U.S. Pat. No. 6,293,031 to Ringbom et al., pressure sensors are arranged between air nozzles adjacent the web to calculate the web tension.
None of these devices directly measures or detects the normal force exerted by the web material on the circumference of the cylinder. Therefore, a more precise, direct and immediate sensing system is needed to provide reliable and constant feedback to an actuation system that maintains an optimal web force against the cylinder circumference.